Fluvial dynamics of dissolved and particulate organic carbon during periodic discharge events in a steep tropical rainforest catchment

Abstract

In small catchments with rapid flood pulses, detailed temporal data are essential because high-discharge events can be measured in hours and days, rather than weeks and months. Using high-resolution (15 min) sampling, we studied the dynamics of aquatic dissolved and particulate organic carbon (DOC and POC) export through episodic discharge events in a small pristine rainforest catchment in northeast Australia between November 2009 and March 2010. High temporal resolution using this instrumentation requires extensive calibration with concurrent field sampling. The concentration of DOC and POC peaked during times of high stream discharge, reflecting an increased mobilization of soil-water carbon stocks. DOC was the major form of organic carbon in the stream (, 70% of the total carbon export). The majority of total organic carbon exported from the catchment (84%) occurred during significant discharge events (discharge . 50 L s 21 ), which occurred only 9% of the time. Export of DOC and POC totaled 195 and 68 kg km 22 month 21 , respectively, with a DOC : POC ratio of 2.9 6 0.9. If this subcatchment was sampled at weekly intervals the lateral export of carbon would have been underestimated by between 49% and 78% for DOC and POC, respectively. Preliminary d 13 C and molar C : N values of the dissolved and particulate matter suggest that during discharge events, less microbially processed material from the upper soil layers dominated organic matter export, with the opposite being true in nonflood conditions. Not only will the quantities of organic matter exported change in different discharge conditions, but the source and quality may also shift. This study reveals that a field-portable instrument for DOC and POC quantification can yield robust, high-temporal-resolution carbon budget estimates, though detailed, site-specific calibration is essential.

The Australian terrestrial carbon budget

“…Bass et al, 2011). These data were used to estimate mean DOC concentrations as follows: the Australian continent is broken up into 246 distinct hydrographic basins (a.k.a.…”

“…Field sampling on a weekly or less frequent basis may miss a significant proportion of the total DOC flux. Bass et al (2011) suggest that insufficient temporal resolution of DOC measurements can lead to underestimating the flux by a factor of two or more.…”

In situand high frequency monitoring of suspended sediment properties using a spectrophotometric sensor

“…To our knowledge, the study of Bass et al (2011) is the only one that used a spectrophotometer to estimate particulate organic matter (POC) at a high temporal resolution and in a catchment. Nonetheless, Bass et al (2011) estimated POC as the difference between TOC and DOC, while no calibration of the instrument was performed to predict POC directly.…”

“…They are often used in wastewater drainage systems for a fast and simple evaluation of wastewater quality (Brito et al, 2014;Gamerith et al, 2011;Rieger et al, 2004;Rieger et al, 2006;Rieger et al, 2008). They have been used to estimate turbidity, dissolved organic carbon (DOC), total organic carbon (TOC) and nitrates (NO 3 -N) in streams at a temporal resolution as high as several minutes (Avagyan et al, 2014;Bass et al, 2011;Grayson and Holden, 2012;Huebsch et al, 2014;Huebsch et al, 2015;Jeong et al, 2012;Koehler et al, 2009;López-Kleine and Torres, 2014;Strohmeier et al, 2013;Waterloo et al, 2006).…”

“…Nonetheless, Bass et al (2011) estimated POC as the difference between TOC and DOC, while no calibration of the instrument was performed to predict POC directly. TOC and DOC were estimated using the internal calibration procedure of the instrument.…”